Humerus (Fragment)
Long Bone of the limb (appendicular) skeleton
Also known as: Upper Arm Bone
Classification
Long Bone of the limb (appendicular) skeleton
Dimensions
In mammals, this is typically a stout and cylindrical bone. Fragments can range from short stubs to larger sections roughly one-fifth to one-third the length of the animal's limb. Proportionally, it is often shorter and thicker than the bones of the lower arm.
Key Features
Look for a thick, cylindrical shaft with a distinctive rounded head at the top that fits into the shoulder. The bottom may feature a pulley-like shape for the elbow joint. On the shaft, there is often a rough, raised area where the shoulder muscles once attached, resembling a small rugged ridge. Even in small fragments, the thick outer wall of solid bone surrounding a spongy, honeycomb-like interior center is a classic identifier.
Similar Bones
This bone is most commonly confused with the Femur (thigh bone). To tell them apart, check the 'neck' of the bone; a femur usually has a distinct, narrow neck leading to a spherical head, whereas the upper arm bone's head sits more directly on top of the shaft. It may also look like a Tibia (shin bone), but the shin bone typically has a much flatter, triangular top.
Across Species
Across the animal kingdom, this bone reflects an animal's lifestyle. In horses, it is short and extremely powerful for sprinting. In whales, it is flattened and shortened into a paddle-like shape. Modern birds have hollow, air-filled versions to save weight during flight, while digging animals like moles have incredibly wide and flared versions to support heavy muscle attachments for moving earth.
Evolutionary History
This bone can be traced back over 350 million years to the earliest lobe-finned fish. As vertebrates moved onto land, what started as a sturdy fin-support evolved into a primary load-bearing pillar for walking. Over millions of years, the shape refined from an L-shaped block in early amphibians to the rounded, efficient lever seen in modern mammals and birds.
Photography Tips
Place the specimen on a neutral, solid-colored background and include a common object like a coin or ruler for size reference. Take photos from several angles: one looking straight down, one from the side to show the curvature, and a close-up of the broken ends to reveal the internal spongy texture.